Three-phase transformers typically include a magnetic core, and three sets of high and low-voltage windings (coils). Each set of high and low-voltage windings is mounted on a respective winding leg of the core.
The windings are typically formed by winding an electrical conductor, such as copper or aluminum wire, on a continuous basis. The electrical conductor can be wound around a mandrel or directly onto an associated winding leg of the transformer. The electrical conductor is wound into a plurality of turns in side by side relationship to form a first layer of turns. A first layer of insulating material is subsequently placed around the first layer of turns. The electrical conductor is wound into a second plurality of turns over the first layer of insulating material, thereby forming a second layer of turns.
A second layer of insulating material is subsequently placed over the second layer of turns. The electrical conductor is then wound into a third plurality of turns over the second layer of insulation, thereby forming a third layer or turns. The above procedures can be repeated until a predetermined number of turn layers have been formed.
The insulating material is typically formed as a sheet or a continuous strip. The insulating material usually includes end fill, i.e., filling material bonded or otherwise secured to opposing sides of the sheet or strip. For example, FIG. 8 depicts a portion of a transformer winding 99 formed using conventional techniques. The transformer winding 99 comprises sheets of insulating material 100 that each include end fill 101, and an electrical conductor 106 wound in layers 108 each formed by a plurality of turns of the electrical conductor 106.
End fill is believed to increase the short-circuit strength of the transformer winding, and can thereby decrease the potential for short-circuit failure. End fill can also inhibit the tendency for the outermost turns of each layer to separate from their adjacent turns and drop down from their respective underlying layers of turns. In other words, the end fill can have a restraining effect that counteracts the tendency of the outermost turns to move outwardly, away from the remaining turns in their respective layers.
The use of end fill can add to the cost of the insulating material (and the overall cost of the transformer winding), can increase the space needed to store the insulating material, and can adversely affect manufacturability of the transformer winding, in comparison to windings formed with insulation that does not include end fill. Moreover, the use of end fill can make it difficult to automate the winding process. The use of insulation with end fill, until recently, was generally considered a necessity in three-phase transformers due to the relatively high kva ratings (50 kva and higher) associated with such transformers (high kva ratings generally necessitate high short-circuit strength). Also, the use of insulation with end fill is often considered necessary to inhibit the tendency of the outermost turns of the transformer winding to separate from their adjacent turns and drop down from their underlying layers, as discussed above.